US20230151468A1

US20230151468A1 - Hot-Rolled Flat Steel Product and Method for the Production Thereof

Info

Publication number: US20230151468A1
Application number: US17/920,533
Authority: US
Inventors: Nicholas Winzer; Ekaterina Bocharova; Roland Sebald
Original assignee: ThyssenKrupp Steel Europe AG
Current assignee: ThyssenKrupp Steel Europe AG
Priority date: 2020-04-22
Filing date: 2020-04-22
Publication date: 2023-05-18
Also published as: CN115427589A; EP4139492A1; WO2021213647A1

Abstract

A hot-rolled flat steel product having a thickness of <1.5 and consisting of, in % by mass, C: 0.04-0.23%, Si: 0.04 0.54%, Mn: 1.4-2.9%, Ti+V, wherein 0.005%<%Ti+%V<0.15%, and optionally one or more elements of Al, Cr, Mo, and B, where AI: 0.01-1.5%, 0.02<%Mo+%Cr<1.4%, and B: 0.0005-0.005%, and the remainder consisting of iron and inevitable impurities. The structure of the flat steel product consists of, in percent by area, in sum, 50-90% ferrite and bainite ferrite, 5-50% martensite, 2-15% residual austenite and <10% other structure elements. The flat steel product has a yield point Rp0.2>290 MPa, a tensile strength Rm>490 MPa and an elongation at break A80 where A80[%]=B−Rm/37 with 31<B<51. To at least one surface of the flat steel product, a Zn coating is applied by hot-dip coating. Also a method for producing a flat steel product of this kind.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is the United States national phase of International Application No. PCT/EP2020/061200 filed Apr. 22, 2020, the disclosure of which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a hot-rolled flat steel product comprising a steel substrate and a corrosion protection layer applied thereto by hot-dip coating on zinc.
In addition, the invention relates to a method for producing such a flat steel product.

Description of Related Art

Flat steel products are understood in the present text as rolled products of which the length and width are each significantly greater than their thickness. These include in particular steel strips and steel sheets.
In the present text, unless explicitly noted otherwise, information about the contents of alloying constituents is always made in % by mass.
The proportions of certain constituents on the structure of the steel substrate of a flat steel product are given in % by area, unless otherwise noted.
In the present text, the term “impurities” of a steel, zinc or other alloy refers to technically unavoidable materials accompanying the steel, which can enter the steel during production or cannot be removed completely therefrom, but the contents of which are in any case so small that they have no influence on the properties of the steel.
The image analysis for the quantitative determination of structure takes place optically by means of light optical microscopy (“LOM”) with 200 to 2000 times and with a scanning electron microscope (“SEM”) with 2000 to 20,000 times magnification.
The distribution of manganese (Mn) in the structure of the steel substrate of a flat steel product according to the invention has been determined by wavelength-dispersive X-ray microrange analysis (WDX) of the structure, which has been described, for example, by Reimer L. (1998) in “Elemental Analysis and Imaging with X-Rays” appearing in Scanning Electron Microscopy, Springer Series in Optical Sciences, vol. 45, Springer, Berlin, Heidelberg.
The strength and expansion properties mentioned here, such as tensile strength Rm, yield point Rp0.2, uniform elongation Ag, elongation A50 and elongation A80 of flat steel products were determined in the tensile test according to DIN-EN 6892-1:2017, unless stated otherwise.
High-load passenger car and truck components, such as crash structures and chassis of automobile bodies, require a hot-dip galvanized steel sheet with a thickness of more than 1.5 mm and a tensile strength of more than 590 MPa.
Frequently, hot-rolled flat steel products that consist of complex phase steels (CP-W), the structure of which consists largely of bainite, are used for the production of such components. However, CP-W steels suffer from relatively lower deformability, which prevents the design of geometrically complex components.
Dual-phase steels (DP), which consist of a combination of hard (e.g., martensite or bainite) and soft (e.g., ferrite) phases, are suitable for complex components due to their combination of high strength and good deformability. However, cold-rolled dual-phase steels (DP-K) with thicknesses greater than 1.5 mm have a higher sensitivity to surface defects, such as ungalvanized areas. Therefore, the maximum sheet thickness of hot-dip galvanized DP-K steels is generally limited to 2 mm.
The direct galvanizing of hot-rolled dual-phase steels (DP-W) is likewise not feasible. For galvanizing, the sheet must be heated to temperatures of greater than 460° C. (the zinc bath temperature). However, at these temperatures, the hard component of the structure, particularly martensite, is tempered and the DP characteristics are lost.
One possibility would be to perform annealing and then galvanizing of a hot-rolled strip in a hot-dip galvanizing line with an annealing cycle typical for DP-K (i.e., partial austenitization in the intercritical temperature range, i.e., in the temperature range lying between the Ac1 and Ac3 temperatures of the respective steel, where α- and γ-Fe are produced in equilibrium). This procedure is similar to the manufacturing process of a DP-K steel except for the cold-rolling step. However, there is the risk here of the omission of the cold-rolling step leading to poorer mechanical properties compared to those of a DP-K steel.
High-strength multi-phase steel with minimum tensile strengths of 580 MPa is known from DE 10 2012 013 113 A1. The steel should preferably have a dual-phase structure and make it possible to produce cold-rolled or hot-rolled steel strips with improved forming properties, from which, in particular, parts for lightweight vehicle construction can be produced. For this purpose, the known multi-phase steel consists of, in % by mass, 0.075%≤C≤0.105%, 0.600%≤Si≤0.800%, 1.000%≤Mn≤2.250%, 0.280%≤Cr≤0.480%, 0.010%≤Al≤0.060%, ≤0.020% P, ≤0.0100% N, ≤0.0150% S and the remainder consisting of iron and impurities.
A further high-strength multi-phase steel with a minimum tensile strength of 580 MPa is the steel known from DE 10 2012 006 017 A1. This should also preferably have a dual-phase structure and be suitable for the production of cold-rolled or hot-rolled steel strips, which have good forming properties. As such, parts for lightweight vehicle construction are to be formed from such steel strips in particular. For this purpose, the known steel consists of, in % by mass, 0.075%≤C≤0.105%, 0.200%≤Si≤0.300%, 1.000%≤Mn≤2.000%, 0.280%≤Cr≤0.480%, 0.010%≤Al≤0.060%, up to 0.020% P, 0.005%≤Nb≤0.025%, up to 0.0100% N, up to 0.0050% S, and the remainder consisting of iron and technically unavoidable impurities.
The steel known from DE 10 2013 013 067 A1 is also among the type of known multi-phase steels explained above, which preferably have a dual-phase structure and are intended to be suitable for cold-rolled or hot-rolled steel strip with improved forming properties. This known steel should have a yield-to-tensile ratio of not more than 73% and, in % by mass, 0.075%≤C≤0.105%, 0.600%≤Si≤0.800%, 1.000%≤Mn≤1.900%, 0.100%≤Cr≤0.700%, 0.010%≤Al≤0.060%, 0.0020%≤N≤0.0120%, ≤0.0030% S, 0.005%≤Nb≤0.050%, 0.005%≤Ti≤0.050%, 0.0005%≤B≤0.0040%, ≤0.200% Mo, ≤0.040% Cu, ≤0.040% Ni and the remainder consisting of iron and unavoidable impurities.

SUMMARY OF THE INVENTION

Against the background of the prior art explained above, the object arises to develop a flat steel product that not only has optimized mechanical properties, but is also particularly suitable for application of a Zn-based corrosion protection layer by hot-dip coating.
The invention has achieved this object by a flat steel product that comprises a steel substrate at least 1.5 mm thick, which consists of, in % by mass, C: 0.04-0.23%, Si: 0.04-0.54%, Mn: 1.4-2.9%, Ti+V, wherein the following applies for the sum %Ti+%V of the contents of Ti and V: 0.005%≤%Ti+%V≤0.15%, and optionally one element or a plurality of the elements from the group “Al, Cr, Mo, B” with the specification that their contents, if present, are dimensioned as follows: Al: 0.01-1.5%, Cr and Mo, wherein the following applies for the sum of the contents %Cr+%Mo of Cr and Mo: 0.02≤%Mo+%Cr≤1.4%, B: 0.0005-0.005%, and the remainder consisting of iron and unavoidable impurities, wherein the unavoidable impurities include less than 0.02% P, less than 0.005% S, less than 0.01% N and less than 0.005% Nb. The flat steel product has a structure which consists of, in % by area, in total 50-90% ferrite and bainitic ferrite, 5-50% martensite, 2-15% residual austenite and up to 10% other structural constituents that are unavoidable due to production, and has a yield point Rp0.2 of at least 290 MPa, a tensile strength Rm of at least 490 MPa and an elongation at break A80 that is determined by the following formula (1): A80[%]=B−Rm/37 with 31≤B≤51. A corrosion protection layer based on zinc is applied to at least one of the surfaces by hot-dip coating
Furthermore, the invention should specify a method with which the production of flat steel products obtained according to the invention is reliably achieved.
To achieve this object, the invention has proposed the method specified in which at least the following work steps are performed. A hot-rolled steel substrate in the form of a steel strip in at least the following sub-steps: A.1) melting a steel melt, which consists of, in % by mass, C: 0.04-0.23%, Si: 0.04-0.54%, Mn: 1.4-2.9%, Ti+V, wherein the following applies for the sum %Ti+%V of the contents of Ti and V: 0.005%≤%Ti+%V≤0.15%, and optionally one element or a plurality of the elements from the group “Al, Cr, Mo, B” with the specification that their contents, if present, are dimensioned as follows: Al: 0.01-1.5%, Cr and Mo, wherein the following applies for the sum of the contents %Cr+%Mo of Cr and Mo: 0.02≤%Mo+%Cr≤1.4%, B: 0.0005-0.005%, and the remainder consisting of iron and unavoidable impurities, wherein the unavoidable impurities include less than 0.02% P, less than 0.005% S, less than 0.01% N and less than 0.005% Nb; A.2) casting the steel melt to form a preliminary product, which is a slab or thin slab; A.3) preheating the preliminary product at a preheating temperature that is at least 1150° C. and at most 1350° C.; A.4) hot-rolling the preliminary product to form a hot-rolled steel strip, wherein the final temperature of the hot rolling is at least 840-980° C. and the thickness of the hot-rolled steel strip is 1.5-10 mm; A.5) cooling the hot-rolled steel strip to a coiling temperature that is 510-640° C.; and A.6) coiling the hot-rolled steel strip cooled to the coiling temperature. The steel substrate, which is present in the form of a hot-rolled steel strip, is coated with a corrosion protection coating based on zinc in at least the following sub-steps, which are passed through continuously: B.1) optional pickling of the hot-rolled steel strip; B.2) heating the hot-rolled steel strip with a heating rate of 0.5-100° C./s to an annealing temperature of 750-950° C. and holding the hot-rolled steel strip at the annealing temperature over an annealing period of 10-1000 s;
B.3) cooling the hot-rolled steel strip at a cooling rate of 0.5-100° C./s to a bath entry temperature BET, for which BT≤BET≤(BT+20° C.) applies, wherein the temperature of the zinc melt bath is referred to as BT, which is 450-480° C.; B.4) passing the hot-rolled steel strip cooled down to the bath entry temperature BET through the zinc melt bath, which consists of up to 5% by mass Mg, up to 10% by mass Al, the remainder of Zn and unavoidable impurities; B.5) cooling the obtained flat steel product with a cooling rate of 0.5-100° C./s; and B.6) optional skin-pass rolling of the flat steel product with a degree of skin passing of 0.3-2.0%. It is self-evident that, when carrying out the method according to the invention, the person skilled in the art does not only carry out the method steps mentioned in the claims and explained here, but also carries out all other steps and activities that are regularly carried out in the prior art upon the practical implementation of such methods, if the necessity arises for this.
Advantageous embodiments of the invention are are explained in detail below, as is the general inventive concept.

DESCRIPTION OF THE INVENTION

The invention thus provides a hot-rolled flat steel product that comprises a steel substrate and a corrosion protection layer based on zinc (Zn) applied thereto by hot-dip coating.
Thereby, the steel of the steel substrate of a flat steel product according to the invention consists of, in % by mass,

C: 0.04-0.23%,
Si: 0.04-0.54%,
Mn; 1.4-2.9%,
Ti+V, wherein the following applies for the sum %Ti+%V of the contents of Ti and V: 0.005%≤%Ti+%V≤0.15%,
and optionally one element or a plurality of the elements from the group “Al, Cr, Mo, B” with the specification that their contents, if present, are dimensioned as follows:
Al: 0.01-1.5%
Sum of contents of Cr+Mo: 0.02-1.4%
B: 0.0005-0.005
and the remainder consisting of iron and unavoidable impurities, wherein the unavoidable impurities include less than 0.02% P, less than 0.005% S, less than 0.01% N and less than 0.005% Nb.

Thereby, the steel substrate of a flat steel product according to the invention is at least 1.5 mm thick and has a structure that consists of, in % by area, in total 50-90% ferrite and bainitic ferrite, 5-50% martensite, 2-15% residual austenite and up to 10% other structural constituents that are unavoidable due to production.
At the same time, a flat steel product according to the invention has a yield point Rp0.2 of at least 290 MPa, a tensile strength Rm of at least 490 MPa and an elongation at break A80 that is determined by the following formula (1):
A80[%]=B−Rm/37 where 31≤B≤51.
A flat steel product according to the invention can be produced by passing through at least the following work steps:

A) Producing a hot-rolled steel substrate in the form of a steel strip in at least the following sub-steps:
A.1) Melting of a steel composed according to the specification of the invention;
A.2) Casting the steel melt to form a preliminary product, which is a slab or thin slab;
A.3) Preheating the preliminary product at a preheating temperature that is at least 1150° C. and at most 1350° C.;
A.4) Hot-rolling the preliminary product to form a hot-rolled steel strip, wherein the final temperature of the hot rolling is at least 840-980° C. and the thickness of the hot-rolled steel strip is 1.5-10 mm;
A.5) Cooling the hot-rolled steel strip to a coiling temperature that is 510-640° C.;
A.6) Coiling the hot-rolled steel strip cooled to the coiling temperature.
B) Coating the steel substrate, which is present in the form of a hot-rolled steel strip, with a corrosion protection coating based on zinc in at least the following sub-steps, which are passed through continuously:
B.1) Optional pickling of the hot-rolled steel strip;
B.2) Heating the hot-rolled steel strip with a heating rate of 0.5-100° C./s to an annealing temperature of 750-950° C. and holding the hot-rolled steel strip at the annealing temperature over an annealing period of 10-1000 s;
B.3) Cooling the hot-rolled steel strip at a cooling rate of 0.5-100° C./s to a bath entry temperature BET, for which BT≤BET≤(BT+20° C.) applies, wherein the temperature of the zinc melt bath is referred to as BT, which is 450-480° C.;
B.4) Passing the hot-rolled steel strip cooled down to the bath entry temperature BET through the zinc melt bath, which consists of up to 5% by mass Mg, up to 10% by mass Al, the remainder of Zn and unavoidable impurities;
B.5) Cooling the obtained flat steel product with a cooling rate of 0.5-100° C./s;
B.6) Optional skin-pass rolling of the flat steel product with a degree of skin passing of 0.3-2.0%.

A preheating temperature of at least 1150° C. is necessary in work step A.1 in order to completely homogenize the structure of the preliminary product. At lower temperatures, the microstructure of the preliminary product would be inherited by the hot strip subsequently produced, so that the Mn segregations desired according to the invention could not be formed. Likewise, at lower preheating temperatures, the alloying elements would be bound in deposits, so that their effects on the mechanical properties of a flat steel product according to the invention could not develop.
A hot-rolling end temperature of at least 840° C. is required to be able to roll the preliminary product alloyed according to the invention in a reliable manner to form a hot-rolled steel strip. At lower hot-rolling end temperatures, the rolling forces would be too high and, as a result, the risk of damage to the rolls of the roll stands used for hot rolling would increase disproportionately. In order to minimize such risks, a hot-rolling end temperature of at least 880° C. can be provided. The hot-rolling end temperature should not exceed 980° C., since hot-rolling end temperatures lying above this upper limit cannot be realized in practice.
The hot-rolled steel strip according to the invention must be at least 1.5 mm thick, so that the Mn segregations desired according to the invention can form in the structure after hot rolling. With smaller strip thicknesses, the hot-rolled steel strip would experience excessively strong deformations during hot rolling, which in turn would result in an undesired homogenization of the Mn distribution in the structure of the hot-rolled steel strip. A steel strip with a thickness of more than 10 mm cannot be used for the intended use. Therefore, the maximum strip thickness is limited to 10 mm.
The coiling temperature at which the hot-rolled steel strip, which forms the steel substrate of the flat steel product according to the invention, is coiled is at least 510° C. in order to secure the formation of Mn segregations during the cooling of the hot-rolled steel strip in the coil. Higher coiling temperatures can promote this process, such that coiling temperatures of at least 530° C., in particular at least 550° C., are particularly advantageous. At excessively low coiling temperatures, an undesired homogeneous Mn distribution would result, with which the mechanical properties desired according to the invention would not be achieved. An excessively high coiling temperature would trigger the risk of pronounced grain boundary oxidation. To prevent this, the coiling temperature is limited to 640° C., preferably 620° C.
After cooling in the coil, the hot-rolled steel strip can, if necessary, be pickled in a conventional manner, in order to remove scale present on the steel strip or to prepare the surface of the steel strip for the work steps carried out below.
For the hot-dip coating, the hot-rolled steel strip is first heated to an annealing temperature at a heating rate of 0.5-100° C. per second in a pre-heating stage. The heating rate must lie within this window in order to ensure sufficient conversion of the structure, in particular its complete recrystallization. For the same reason, an annealing temperature of 750-950° C. and a holding time of 10-1000 seconds are required. At excessively low annealing temperatures or excessively short holding times, the structure would not crystallize completely with the result that, during the subsequent cooling, insufficient austenite would be available to form the desired martensite proportion of the structure. An unrecrystallized steel substrate would also result in a pronounced anisotropy of the mechanical properties of a flat steel product according to the invention.
Cooling from the annealing temperature to the zinc bath entry temperature BET likewise takes place at a cooling rate of 0.5 to 100° C. per second. The bath entry temperature BET is at least equal to and at most 20° C. higher than the melt bath temperature, in order to prevent the melt bath temperature from changing substantially by the entry of the hot-rolled steel strip.
Optionally, a further heat treatment (“galvannealing”) can follow the hot-dip coating, with which the hot-dip coated flat steel product is heated up to 550° C., in order to burn in the previously applied corrosion protection layer.
Either immediately after the exit from the zinc bath or after the additional heat treatment, the flat steel product obtained is cooled to room temperature at a cooling rate of 0.5-100° C./s.
The flat steel product thus produced can optionally be subjected to a conventional skin-pass rolling, in order to optimize its dimensional accuracy and surface properties. The degree of skin passing set here is typically at least 0.3% and at most 2.0%, wherein degrees of skin passing of at least 0.5% have proven to be particularly practical. A degree of skin passing of less than 0.3% leads to a lower surface roughness of the corrosion protection layer, which would have a negative influence on the formability of the flat steel product. At a degree of skin passing of more than 2.0%, the yield point Rp0.2 is increased and the elongation at break A80 is reduced, so that an elongation at break according to formula 1 could not be achieved.
Surprisingly, it has been found that a flat steel product that comprises a steel substrate that is alloyed according to the invention and has a structure according to the invention achieves high elongation at break values in the hot-rolled state, which are comparable with the elongations at break A80, which conventionally cold-rolled flat steel products of the type explained at the outset have (“DP-K steels”), which have similar strengths. Thus, in practice, elongation at break values A80 can regularly be achieved, for which the parameter B in formula (1) is at least in the range 31-51, preferably 36-46.
The combination of high strength and high elongation at break values results from the proportion of 2-15% by area residual austenite present in the steel substrate of a flat steel product according to the invention, wherein residual austenite proportions of at least 5% by area are regularly present in the structure of the steel substrate of a flat steel product according to the invention and have a positive effect on the mechanical properties of the flat steel product. The residual austenite contents that can be established in the flat steel product according to the invention are thus significantly higher than with a cold-rolled flat steel product with a comparable alloy.
According to the findings of the invention, the presence of larger residual austenite proportions in the structure is a result of the inheritance of Mn segregations that are present in the steel substrate, hot-rolled according to the invention, of a flat steel product according to the invention and that are maintained via the annealing treatment, which the flat steel product passes through for its hot-dip coating. It could thus be shown that, in the manner according to the invention of producing a flat steel product according to the invention after coiling (sub-step A.6 of the method according to the invention) and before hot-dip coating (work step B of the method according to the invention), the hot-rolled steel substrate has a highly anisotropic and inhomogeneous structure with a high pearlite content, which is present in line form. Wavelength-dispersive X-ray microrange analyses (WDX) of the structure result in the fact that Mn in the pearlite lines is segregated and the Mn segregations are present in a highly anisotropic and inhomogeneous distribution after coiling and before hot-dip coating.
With the hot-dip coating taking place in a continuous pass, the steel substrate of a flat steel product according to the invention passes through an annealing (sub-step B.2 of the method according to the invention) before entry into the melt bath, during which it is kept at the annealing temperature over a period of time. Thereby, according to the invention, the annealing temperature and the annealing duration are coordinated with one another in such a manner that there is no redistribution of the Mn segregations. Therefore, in the case of the finished hot-dip coated flat steel product according to the invention, despite the annealing treatment required for the preparation of the Zn corrosion protection coating, an anisotropic and inhomogeneous Mn distribution in the steel substrate is also present, which, as such, has been “inherited” from the end structure present after the coiling of the hot-rolled steel substrate of the flat steel product.
Given that Mn contributes very strongly to the stability of the austenite during the annealing in the intercritical region, both the conversion temperature and the residual austenite content after cooling are distributed in a more inhomogeneous manner in comparison to hot-rolled flat steel products that were coiled at lower temperatures in deviation from the specification of the invention. With a flat steel product produced according to the invention, the structural regions of the steel substrate in which there is a higher Mn concentration transform more easily and thus retain more austenite after cooling than the structural regions in which a lower Mn concentration is present. They convert at higher temperatures or not at all, whereby a higher proportion of the original ferrite is maintained there.
The inhomogeneity of the Mn distribution in the steel substrate of a fully processed flat steel product according to the invention can be quantified by the total surface proportion of the structure of the steel substrate in which an Mn concentration (in % by mass) is present which is more than 15% higher than the average value of the Mn concentrations in the entire structure of the flat steel product. The sum of the surface proportions of the structure of the steel substrate of a flat steel product according to the invention which have an Mn concentration that is more than 15% higher than the average value of the Mn concentration in the entire structure is referred to as “X.” In a flat steel product according to the invention, X is at least 10%, in particular at least 12%, advantageously at least 15% of the total structure. The surface proportions forming the sum X can be evaluated using a WDX measurement, wherein typically the Mn concentration is determined over a measurement surface of at least 200×200 μm with a step size of 0.5 μm.
The steel of the steel substrate of a flat steel product according to the invention present in the course of the production according to the invention as a hot-rolled steel strip is composed as follows:
Carbon (C) is present in the steel substrate of a flat steel product according to the invention in contents of 0.04-0.23% by mass. C is an essential element for the formation of martensite and austenite, which are required in order to achieve the strength properties required by a flat steel product according to the invention. In order for this effect to occur to a sufficient extent, the steel according to the invention contains at least 0.04% by mass, wherein the desired effect is achieved particularly reliably at C contents of at least 0.07% by mass. An excessively high C content would have a negative effect on the welding behavior of the flat steel product. In general, the weldability of a steel decreases with the level of its C content. In order to avoid negative influences of the C content on its processability, the C content of the steel according to the invention is therefore limited to a maximum of 0.23% by mass, in particular to a maximum of 0.20% by mass, wherein the negative effects of the presence of C can be particularly reliably avoided at contents of at most 0.17% by mass.
Silicon (Si) is present in the steel substrate of a flat steel product according to the invention in contents of 0.04-0.54% by mass. Si is required to suppress the formation of pearlite in the structure during annealing, which would have a negative effect on the mechanical properties of the end product. A minimum content of 0.04% by mass Si is required for this purpose. An excessively high Si content also prevents the formation of pearlite during coiling and thus the segregation of Mn in the structure of the steel substrate. A significant segregation of Mn during coiling is necessary to achieve a high sum X and the desired mechanical properties. An excessively high Si content would likewise impair the surface quality of a flat steel product according to the invention. For these reasons, the upper limit of the Si content is limited to 0.54% by mass.
Aluminum (Al) can optionally be added to the steel substrate of a flat steel product according to the invention in contents of 0.01-1.5% by mass, in order to contribute to the suppression of the formation of pearlite. Even if Al is used in the usual manner for deoxidation of the melt, a minimum Al content of 0.01% by mass results. However, an excessively high Al content can have a negative effect on the castability of the steel and worsen the coating behavior during the hot-dip coating. Such negative influences of the presence of Al in the steel of the substrate of a flat steel product according to the invention can thereby be avoided particularly reliably in that the Al content is limited to at most 1.0% by mass, in particular at most 0.5% by mass.
Manganese (Mn) is present in the steel substrate of a flat steel product according to the invention in contents of 1.4-2.9% by mass. Mn is a mixed crystal element that contributes to the strength of the material. The presence of Mn in the steel of the substrate of a flat steel product according to the invention additionally stabilizes the austenite in the structure of the substrate. The special feature of the alloy concept according to the invention in combination with the production according to the invention of a flat steel product according to the invention consists in that a flat steel product according to the invention is an optimal combination of high tensile strength and high elongation at break as a result of the segregation of Mn in the pearlite lines of the steel substrate after coiling, which is also maintained if the flat steel product has been annealed for the hot-dip coating and has passed through the hot-dip bath. In order for Mn to be enriched to a sufficient degree in the pearlite lines by segregation, Mn contents of at least 1.4% by mass are required, wherein it is favorable with regard to the reliability with which the positive influence of Mn on the properties of a flat steel product according to the invention is established when the Mn content is at least 1.5% by mass. However, an excessively high Mn concentration would also have a negative effect on weldability. Therefore, the upper limit of the Mn content of the steel substrate of a flat steel product according to the invention is limited to 2.9% by mass, preferably 2.5% by mass, wherein the amount of Mn for the properties of a flat steel product according to the invention can be utilized particularly effectively at Mn contents of up to 2.2% by mass.
Chromium (Cr) and molybdenum (Mo) can be added to the steel of the steel substrate of a flat steel product according to the invention as optional elements for increasing strength. In addition, the presence of Cr and/or Mo increases the formation of martensite with respect to pearlite during the cooling of the flat steel product from the intercritical region in a continuous coating line. If these effects are to be utilized, contents of Cr and Mo that in total amount to at least 0.02% by mass, in particular at least 0.05% by mass, are required. In the case of excessively high Cr contents, however, the risk of pronounced grain boundary oxidation would be increased. An excessively high Mo content is also to be avoided for reasons of cost. In order to be able to effectively utilize the effects of Cr and Mo in the steel of the steel substrate of a flat steel product according to the invention, the upper limit of the total content of Cr and Mo is therefore set to 1.4% by mass, preferably 1.0% by mass. Thereby, Cr and Mo do not necessarily have to occur in combination, but can also each be added alone to the steel in contents of 0.02 to 1.4% by mass, in particular 0.05-1.0% by mass, as specified according to the invention, in order to achieve the effects explained. However, particularly favorable effects result when Cr and Mo are present together, each in effective contents, as long as the sum of such contents is within the limits according to the invention.
At least one of the elements of titanium (Ti) and vanadium (V) is present as a required constituent in the steel of the steel substrate of a flat steel product according to the invention in contents of 0.005-0.15% by mass, wherein, here as well, it applies that an optimal effect of such elements occurs when Ti and V are each present together in effective contents. Ti and V are micro-alloying elements that cause the formation of fine precipitates in the steel. Such precipitates prevent the coarsening of the austenite grains at temperatures that are higher than the Ar1 temperature of the steel, and in this manner lead to the refinement of the structure. A finer structure favors the segregation of Mn that is desired according to the invention during the coiling carried out in the course of the production of a flat steel product according to the invention, because the distance over which Mn diffuses is reduced by the presence of Ti and/or V. Ti-containing and V-containing precipitates also contribute to the strength of a flat steel product according to the invention by dispersion hardening. To achieve these effects of Ti and V, Ti and/or V contents of at least 0.005% by mass in total are required. At contents above 0.15% by mass, the presence of Ti and/or V no longer results in any particular increase with regard to the properties desired according to the invention. Rather, Ti and V can be utilized particularly effectively if the sum of their contents is at most 0.1% by mass.
According to the invention, the content of niobium (Nb) is limited to less than 0.005% by mass, so that, if niobium is present at all, it is among the impurities that are technically ineffective. Higher Nb contents would lead to the formation of fine Nb precipitates, which would bring about susceptibility to crack formation during continuous casting or in the case of the slab cooling or reheating. Therefore, the Nb content is preferably limited to less than 0.003% by mass, in particular less than 0.002% by mass.
Boron (B) can likewise optionally be added to the steel of the steel substrate of a flat steel product according to the invention in contents of 0.0005-0.005% by mass, in order to prevent the formation of ferrite from the intercritical region in the course of the cooling carried out during the production of the flat steel product. In this manner, B promotes the formation of bainite, which leads to an increase in strength. For this purpose, a minimum content of 0.0005% by mass B is required, but excessively high B content can lead to undesired embrittlement. Therefore, according to the invention, the upper limit of the B content, if B is added, is set to not more than 0.005% by mass, in particular 0.002% by mass.
Phosphorus (P) is among the undesired, but technically generally unavoidable impurities of the steel of the steel substrate of a flat steel product according to the invention and should therefore be as low as possible. P proves to be disadvantageous in particular with regard to weldability. In order to reliably avoid its unfavorable influence, the P content according to the invention is limited to less than 0.02% by mass, preferably less than 0.01% by mass, in particular less than 0.005% by mass.
Sulfur (S) is also among the undesired, but technically generally unavoidable impurities of the steel of the steel substrate of a flat steel product according to the invention and should therefore be as low as possible. At higher concentrations, S leads to the formation of MnS or (Mn, Fe)S, which would have a negative effect on the elongation behavior of a flat steel product according to the invention. In order to avoid such unfavorable effects, the S content according to the invention is limited to less than 0.005% by mass, preferably less than 0.002% by mass.
Nitrogen (N) also includes the undesired, but technically generally unavoidable impurities of the steel of the steel substrate of a flat steel product according to the invention and should therefore be as low as possible. N forms, for example, nitrides with aluminum or titanium. In the case of higher N contents, this would lead to coarse precipitates that could be harmful to the formability of the flat steel product. Therefore, the N content is limited according to the invention to less than 0.01% by mass, preferably less than 0.005% by mass.
In conventional steel production, calcium (Ca) also enters the steel because it is added both for deoxidation and desulfurization and to improve castability. An excessively high concentration of Ca can lead to the formation of undesired inclusions, which have a negative effect on mechanics and rollability. Therefore, the upper limit of the Ca content is limited to at most 0.005% by mass, preferably at most 0.002% by mass,
Copper (Cu), nickel (Ni), tin (Sn), arsenic (As), cobalt (Co), zirconium (Zr), lanthanum (La) and/or cerium (Ce) are alloying elements that are also among the impurities of the steel of the steel substrate of a flat steel product according to the invention, the presence of which is undesirable per se. In order to reliably prevent influences of such elements on the properties of a flat steel product according to the invention, in the steel of the steel substrate of a flat steel product according to the invention, the Cu content is limited to not more than 0.2% by mass, the Ni content is limited to not more than 0.1% by mass, the Sn content is limited to not more than 0.05% by mass, the As content is limited to not more than 0.02% by mass, the Co content is limited to not more than 0.02% by mass, the Zr content is limited to not more than 0.0002% by mass, the La content is limited to not more than 0.0002% by mass, and the Ce content is limited to not more than 0.0002% by mass.
Oxygen (O) is also an undesirable impurity, since in the presence of larger amounts of O, oxide deposits are formed, which have a negative effect both on the mechanical properties of the flat steel product and on the castability and rollability of the steel of its steel substrate. Therefore, the content of oxygen is limited to at most 0.005% by mass, preferably 0.002% by mass.
Hydrogen (H) is also among the undesirable impurities of the steel of the steel substrate of a flat steel product according to the invention. As the smallest atom, H is highly mobile on interstitial sites in the steel and can lead to cracking in the core during cooling from hot rolling, in particular in ultrahigh-strength steels. Therefore, the content of H in the steel of the steel substrate of a flat steel product according to the invention is reduced to a maximum of 0.001% by mass, preferably a maximum of 0.0006% by mass, more preferably a maximum of 0.0004% by mass, most preferably a maximum of 0.0002% by mass.
No particular requirements are imposed on the composition of the corrosion protection coating and the associated melt bath through which the flat steel product passes during its hot-dip coating. Thus, the corrosion protection coating of a flat steel product according to the invention consists of zinc (Zn) in its main proportion and can otherwise be composed in a conventional manner.
Accordingly, in addition to Zn and unavoidable impurities, the corrosion protection layer can contain up to 20% by mass Fe, up to 5% by mass Mg and up to 10% by mass Al. Typically, if they are each present, at least 5% by mass Fe, at least 1% by mass Mg and/or at least 1% by mass Al is provided, in order to achieve optimal usage properties of corrosion protection.
The invention is explained in more detail below with reference to exemplary embodiments.
To test the invention, steels A-1 were melted and cast into slabs, the composition of which is specified in Table 1. Contents of an alloying element that are so small that they are “0” in the technical sense, i.e., are so small that they have no influence on the properties of the steel, are referred to in Table 1 by the entry “-”.
The slabs were heated through in a preheating furnace in which a preheating temperature VT prevailed.
Subsequently, the preheated slabs were hot rolled in a conventional manner to form hot-rolled steel strips W1-W35, wherein the hot rolling was ended at an end rolling temperature ET.
The hot-rolled steel strips W1-W35 obtained in this manner were coiled in a likewise conventional manner starting from a coiling temperature HT in a likewise conventional manner to each form a coil. If necessary, they were cooled to the coiling temperature HT in a conventional manner for this purpose before coiling.
To demonstrate the effect of the invention, in the production of the hot-rolled steel strips W1-W35, which consisted of one of the steels A-1 in each case, one of the combinations I-VIII specified in Table 2 of pre-heating furnace temperature VT, hot-rolling end temperature ET and coiling temperature HT was selected in each case. The preheating furnace temperatures VT, hot-rolling end temperatures ET and coiling temperatures HT belonging to each of the combinations I-VIII are specified in Table 2. Thereby, those preheating furnace temperatures VT, hot-rolling end temperatures ET and coiling temperatures HT which in each case do not correspond to the specifications of the invention are emphasized by underlining.
After cooling in the coil, the hot-rolled steel strips W1-W35 were coated with a Zn-based corrosion protection layer by hot-dip coating. For this purpose, they were subjected in each case to one of six variants a-f of an annealing treatment and a melt application, in which they were heated in a pre-heating stage with a heating rate HR to an annealing temperature GT, at which they were subsequently held over an annealing period of 40 s to 100 s in each case. Subsequently, the hot-rolled steel strips W1-W35 were cooled with a cooling rate KR1 to a bath entry temperature BET, which was in each case equal to the bath temperature of the melt bath, through which the hot strips were passed after the respective annealing treatment a-f. The melt bath consisted of at least 99% by mass Zn. The now complete flat steel products emerging from the melt bath and produced on the basis of the hot-rolled steel strips W1-W35 were subsequently cooled to room temperature at a cooling rate KR2. The parameters of heating rate HR, annealing temperature GT, cooling rate KR1, bath entry temperature BET and cooling rate KR2 belonging to the variants a-f of the annealing treatment and the melt application are recorded in Table 3.
The mechanical properties and structural constituents of the flat steel products obtained in the manner explained above were determined. The results of these investigations of yield point Rp0.2, tensile strength Rm, elongation at break A80, parameter “B” from formula (1), ferrite proportion F of the structure, martensite proportion M of the structure, austenite proportion A of the structure, proportion SO of the other constituents of the structure and sum X of the surface proportions of the structure of the steel substrate, in which there is a Mn concentration that is more than 15% above the average value of the Mn concentration in the structure are summarized in Table 4, where, for the flat steel products produced on the basis of the hot-rolled steel strips W1-W35, it is also specified which of the steels A-1 the steel substrate of the respective flat steel product consisted of, and which of the combinations I-VIII of hot strip production (“WEZ” column) and which of the variants a-f of the annealing treatment and the melt application the respective steel substrate passed through (“GS” column).
The flat steel products produced from the hot-rolled steel strips W1, W3, W6, W7, W8 and W27 were not produced in accordance with the invention:
With the flat steel product produced from the hot-rolled steel strip W1, the slab was heated with an excessively low preheating temperature VT, so that the slab was not fully annealed. As a result, the alloying elements and the production processes did not affect the mechanical properties.
The hot-rolled steel strip W3 contains too little Mn, so that Mn in the pearlite lines of the hot strip structure did not segregate to sufficient degrees. This resulted in a lower residual austenite content and therefore to a relatively low elongation at break A80 of the flat steel product produced from the hot-rolled steel strip W3. As a result, parameter B was below 31.
In the production of the hot-rolled steel strips W6, W7 and W8, excessively low coiling temperatures were set. This led to a similar effect on the Mn segregations and therefore to insufficient mechanical properties, as in the flat steel product produced from the hot-rolled steel strip W3.
During the annealing treatment of the hot-rolled steel strip W27, an excessively low GT was set, so that the structure was not completely recrystallized. This resulted in a low austenite content in the structure of the steel substrate of the flat steel product obtained and therefore to a low elongation at break A80.

TABLE 1

Content information in % by mass
Remainder of iron and unavoidable impurities

Steel	C	Si	Al	Mn	Cr + Mo	Ti + V	Nb	B	P	S	N

A	0.043	0.052	0.025	1.41	0.02	0.008	0.003	—	0.004	0.004	0.003
B	0.051	0.061	0.023	1.23	0.139	0.011	0.004	—	0.004	0.003	0.004
C	0.071	0.107	0.021	1.51	0.043	0.038	0.004	0.0018	0.005	0.002	0.004
D	0.087	0.042	0.172	1.92	0.350	0.057	0.002	—	0.007	0.003	0.003
E	0.142	0.053	0.185	1.62	0.396	0.031	0.002	0.0012	0.004	0.004	0.003
F	0.150	0.487	0.016	2.00	0.329	0.051	0.003	—	0.008	0.004	0.003
G	0.168	0.193	0.980	2.15	0.694	0.091	0.004	—	0.009	0.002	0.004
H	0.192	0.048	1.46	2.41	0.986	0.105	0.004	—	0.012	0.003	0.007
I	0.222	0.54	0.093	2.76	1.367	0.143	0.002	0.0046	0.018	0.004	0.009

TABLE 2

Temperature information in ° C., underlined
values are not according to the invention

Combination	VT	ET	HT	According to the invention?

I	1100	850	550	NO *)
II	1150	890	500	NO *)
III	1200	910	520	YES
IV	1250	930	540	YES
V	1150	890	560	YES
VI	1200	910	590	YES
VII	1250	930	610	YES
VIII	1150	890	630	YES

*) Parameters not according to the invention are underlined

TABLE 3

Underlined values are not according to the invention.

	HR	GT	KR1	BT	KR2
Variant	[° C./sec]	[° C.]	[° C./sec]	[° C.]	[° C./sec]

a	7	725	4	455	6
b	12	750	5	465	8
c	23	775	7	460	11
d	41	800	10	470	13
e	60	825	14	465	16
f	73	850	18	475	19

TABLE 4

Rp0.2	Rm	A80	F	M	A	SO	X

Strip	Steel	WEZ	GS	[MPa]	[%]	B	[% by mass]	[%]

W1	A	I	e	368	523	16.5	30.6	70.0	10.0	1.5	18.5	13.7
W2	A	III	e	311	503	28.9	42.5	80.0	15.0	3.5	1.5	10.5
W3	B	III	e	286	483	17.3	30.4	85.0	12.0	1.0	2.0	12.1
W4	C	III	c	340	569	25.7	41.1	77.0	16.0	4.5	2.5	11.6
W5	C	III	f	348	675	25.9	44.1	73.0	20.0	5.0	2.0	10.2
W6	D	II	b	432	754	9.6	30.0	65.0	32.0	1.5	1.5	8.4
W7	D	II	d	364	687	10.9	29.5	64.0	33.0	1.0	2.0	9.9
W8	D	II	f	363	708	11.3	30.4	62.0	35.0	1.5	1.5	8.9
W9	D	V	b	498	783	13.5	34.7	67.0	27.0	3.0	3.0	13.8
W10	D	V	d	475	580	23.1	38.8	76.0	20.0	2.5	1.5	14.2
W11	D	V	f	351	700	20.3	39.2	71.0	25.0	2.5	1.5	12.9
W12	D	VIII	b	385	681	24.6	43.0	70.0	22.0	6.0	2.0	17.8
W13	D	VIII	d	352	661	26.7	44.6	68.0	24.0	6.0	2.0	18.5
W14	D	VIII	f	343	673	26.6	44.8	71.0	21.0	5.0	3.0	19.3
W15	E	III	b	615	1030	14.5	42.3	58.0	38.0	2.0	2.0	13.2
W16	E	III	d	581	995	15.3	42.2	58.0	37.0	2.5	2.5	12.4
W17	E	III	f	870	1190	9.0	41.2	56.0	40.0	2.0	2.0	12.9
W18	E	VI	b	416	756	17.3	37.7	67.0	28.0	3.0	2.0	17.1
W19	E	VI	d	423	787	21.2	42.5	64.0	27.0	6.5	2.5	16.8
W20	E	VI	f	503	839	22.1	44.8	63.0	27.0	7.5	2.5	17.7
W21	F	IV	b	509	947	13.2	38.8	62.0	30.0	6.0	2.0	12.2
W22	F	IV	d	431	875	15.9	39.5	65.0	26.0	7.5	1.5	12.4
W23	F	IV	f	423	858	15.6	38.8	65.0	26.0	7.0	2.0	13.5
W24	F	VII	b	510	921	16.7	41.6	60.0	28.0	10.0	2.0	16.8
W25	F	VII	d	407	813	22.8	44.8	66.0	21.0	10.0	3.0	17.8
W26	F	VII	f	420	801	23.2	44.8	68.0	21.0	8.5	2.5	18.2
W27	G	IV	a	382	779	9.5	30.6	65.0	20.0	0.5	14.5	13.5
W28	G	IV	c	489	892	19.6	43.7	65.0	23.0	9.5	2.5	12.2
W29	G	IV	e	445	822	22.9	45.1	67.0	21.0	10.5	1.5	12.7
W30	H	V	b	475	875	14.4	38.0	68.0	25.0	4.5	2.5	15.1
W31	H	V	d	519	875	20.8	44.4	63.0	24.0	11.0	2.0	15.9
W32	H	V	f	613	951	17.7	43.4	63.0	23.0	12.0	2.0	15.6
W33	I	VI	b	600	1030	14.8	42.6	55.0	29.0	13.0	3.0	16.2
W34	I	VI	d	569	981	18.2	44.7	55.0	27.0	14.5	3.5	16.3
W35	I	VI	f	851	1190	7.3	39.5	49.0	35.0	14.0	2.0	17.5

Underlined values are not according to the invention.

Claims

1. A hot-rolled flat steel product, which comprises

a steel substrate at least 1.5 mm thick,

which consists of, in % by mass,

C: 0.04-0.23%,

Si: 0.04-0.54%,

Mn: 1.4-2.9%,

Ti+V, wherein the following applies for the sum %Ti+%V of the contents of Ti and V:

0.005%≤%Ti+%V≤0.15%,

and optionally one element or a plurality of elements selected from the group consisting of Al, Cr, Mo, and B, wherein:

AI: 0.01-1.5%,

the sum of the contents %Cr+%Mo of Cr and Mo: 0.02≤%Mo+%Cr≤1.4%, and

B: 0.0005-0.005%,

and a remainder consisting of iron and unavoidable impurities, wherein the unavoidable impurities include less than 0.02% P, less than 0.005% S, less than 0.01% N and less than 0.005% Nb,

wherein the hot-rolled flat steel product has a structure which consists of, in % by area, in total 50-90% ferrite and bainitic ferrite, 5-50% martensite, 2-15% residual austenite and up to 10% other structural constituents that are unavoidable due to production,

has a yield point Rp0.2 of at least 290 MPa, a tensile strength Rm of at least 490 MPa and an elongation at break A80 determined by the following formula (1):

A80[%]=B−Rm/37 with 31≤B≤51, and

a corrosion protection layer based on zinc is applied to at least one surface of the steel substrate by hot-dip coating.

2. The flat steel product according to claim 1, wherein the structure of the steel substrate contains at least 5% by area residual austenite.

3. The flat steel product according to claim 1, wherein, for the parameter B of formula (1), the following applies: 36≤B≤46.

4. The flat steel product according to claim 1, wherein the sum X of surface proportions of the structure of the steel substrate in which there is a Mn concentration that is more than 15% above the average value of the Mn concentration in the structure is at least 10% of the total structure.

5. The flat steel product according to claim 4, wherein the sum X is at least 12%.

6. The flat steel product according to claim 5, wherein the sum X is at least 15%.

7. The flat steel product according to claim 1, wherein the corrosion protection layer contains at least 75% by mass Zn.

8. A method for the production of a flat steel product according to claim 1 comprising:

A) producing a hot-rolled steel substrate in the form of a steel strip in at least the following sub-steps:

A.1) melting a steel melt, which consists of, in % by mass, C: 0.04-0.23%, Si: 0.04-0.54%, Mn: 1.4-2.9%, Ti+V, wherein the following applies for the sum %Ti+%V of the contents of Ti and V: 0.005%≤%Ti+%V≤0.15%, and optionally one element or a plurality of the elements selected from the group consisting of Al, Cr, Mo, and B, wherein AI: 0.01-1.5%, the sum of the contents of Cr and Mo: 0.02≤%Mo+%Cr≤1.4%, and B: 0.0005-0.005%, and a remainder consisting of iron and unavoidable impurities, wherein the unavoidable impurities include less than 0.02% P, less than 0.005% S, less than 0.01% N and less than 0.005% Nb;

A.2) casting the steel melt to form a preliminary product, which is a slab or thin slab;

A.3) preheating the preliminary product at a preheating temperature of at least 1150° C. and at most 1350° C.;

A.4) hot-rolling the preliminary product to form a hot-rolled steel strip, wherein the final temperature of the hot rolling is at least 840-980° C. and the thickness of the hot-rolled steel strip is 1.5-10 mm;

A.5) cooling the hot-rolled steel strip to a coiling temperature that is 510-640° C.; and

A.6) coiling the hot-rolled steel strip cooled to the coiling temperature; and

B) coating the steel substrate, which is present in the form of a hot-rolled steel strip, with a corrosion protection coating based on zinc in at least the following sub-steps, which are passed through continuously:

B.1) optional pickling of the hot-rolled steel strip;

B.2) heating the hot-rolled steel strip with a heating rate of 0.5-100° C./s to an annealing temperature of 750-950° C. and holding the hot-rolled steel strip at the annealing temperature over an annealing period of 10-1000 s;

B.3) cooling the hot-rolled steel strip at a cooling rate of 0.5-100° C./s to a bath entry temperature BET, for which BT≤BET≤(BT+20° C.) applies, wherein BT is the temperature of the zinc melt bath and is 450-480° C.;

B.4) passing the hot-rolled steel strip cooled down to the bath entry temperature BET through the zinc melt bath, which consists of up to 5% by mass Mg, up to 10% by mass AI, and a remainder of Zn and unavoidable impurities to obtain a flat steel product;

B.5) cooling the obtained flat steel product with a cooling rate of 0.5-100° C./s; and

B.6) optional skin-pass rolling of the flat steel product with a degree of skin passing of 0.3-2.0%.

9. The method according to claim 8, wherein the coiling temperature is at least 530° C.

10. The method according to claim 9, wherein the coiling temperature is at least 550° C.

11. The method according to claim 8, wherein the coiling temperature is at most 620° C.